In response section 6:
(So that I can reference, the full section:)
6. Each time you redistribute the Program (or any work based on the
Program), the recipient automatically receives a license from the
original licensor to copy, distribute or modify the Program subject to
these terms and conditions. You may not impose any further restrictions
on the recipients' exercise of the rights granted herein. You are not
responsible for enforcing compliance by third parties to this License.
It seems to me that the license from the original licensor would
include this new term/condition, as that is how (s)he licensed it.
Perhaps a rephrase as "an additional term" or something that made it
obvious that this was a part of the license of the program and hence
is not an additional restriction when redistributed? I of course
can't make an entirely new license based on the GNU GPL without FSF's
permission, so is there any way that a term could be added at all?